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The Transit spacecraft were developed for updating the inertial navigation
systems on board US Navy Polaris submarines, and later for civilian use.
Transit receivers used the known characteristics of the satellite's orbit,
measured the Doppler shift of the satellite's radio signal, and thereby
calculated the receivers position on the earth. As a single spacecraft
travelled overhead, the user measured the Doppler shift over a 15 minute
period by receiving timing marks and satellite orbital information on two
separate frequencies, 149.99 and 399.97 MHz. These signals were corrected
for ionospheric refraction and the information was then fed into the users
navigation system.

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Individual Transit satellites operated for over 10 years. Technical break-
throughs during the program included gravity gradient stabilization, the use
of radio-isotope thermoelectic generators (RTG), and navigation satellite
technologies later used in the GPS system. Transit was superseded by the
Navstar global positioning system. The use of the satellites for navigation
was discontinued at the end of 1996 but the satellites continued transmitting
and became the Navy Ionospheric Monitoring System (NIMS).
<hr width="25%" noshade><h3>Discipline(s)</h3>
     Navigation & Global Positioning<br>
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<h3>Sponsoring Agencies/Countries</h3>
    National Aeronautics and Space Administration/United States<br>
    Department of Defense-Department of the Navy/United States<br>
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